The invention relates to antennas capable of generating multiple beams through a common aperture.
Radar, communication and electronic warfare systems must often be capable of both transmitting high power signals and receiving low power signals. Another desirable feature is the capability of simultaneously transmitting signals to or receiving signals from a number of geographically separate locations. For example, radar must often provide multiple operating modes, such as search and track, where each mode has different waveform parameters and antenna steering requirements. Communication systems must often maintain links with two or more nodes that are not along the same line of sight. And, electronic warfare systems must often receive and resolve signals over a wide angular field of view while simultaneously transmitting with several narrow beams having different, unrelated directions.
These capabilities can be achieved through the use of multiple independently steerable antenna apertures. Or, if a shared aperture is more desirable, certain beam forming networks may be used. There are two major types of beam forming networks, namely, matrices and lenses.
One example of a matrix network is the Butler matrix, described by J. Butler and R. Lowe in "Beam Forming Matrix Simplifies Design of Electronically Scanned Antenna", Electronic Design, Vol. 9, No. 8, pp. 170-73 (Apr. 12, 1961). The Butler matrix is a linear, bilateral device with the properties of superposition and reciprocity. It has 2.sup.N input ports and 2.sup.N output ports. Typically, each output port is connected to a corresponding element of a linear array of radiating elements. Driving only one input port with a source of electromagnetic energy produces a single beam that has a direction corresponding to the input port that was selected. Driving multiple ports results in multiple beams each having a direction corresponding to the input port that was driven. If all of the input ports are driven, a cluster of 2.sup.N beams results. The cluster of beams may be scanned in space if beam steering elements, such as phase shifters or time delay networks, are placed between every output port of the matrix and its corresponding radiating element. However, the beam steering elements do not permit any single beam to be steered independently of any other beam.
In the Butler matrix, signal parameters, such as center frequency, total bandwidth and modulation, can differ from one input port to another. Thus, different signals can be launched in different directions as long as the beams are orthogonal. Furthermore, when the Butler matrix is operated in a receive only mode, the port from which energy emerges identifies the direction from which the energy was received.
Switching beam directions is accomplished by switching input ports. When the number of simultaneous beams is large and/or when a high power level is being used (as is common in radar, communications and electronic warfare systems) switching input ports can become complicated.
Another matrix beam forming network is the Blass matrix as described by J. Blass in "The Multidirectional Antenna: A New Approach to Stacked Beams", 1960 I.R.E. Convention Record, Pt. 1, pp. 48-50. It differs from the Butler matrix in that neither the input nor output ports are constrained to the quantity 2.sup.N and the number of input and output ports need not be equal. In addition, the beams need not be orthogonal. The Blass matrix forms a cluster of beams that can be rapidly steered electronically, in the same manner as for the Butler matrix, but none of the beams can be steered independently without dynamically revising the design of the matrix.
Lens beam forming networks share the same basic properties of matrices, namely, they are linear, bilateral devices with the properties of superposition and reciprocity. Lens beam forming networks are a class of antenna that is similar to an optical lens, i.e. the microwave lens converts a point source of electromagnetic energy into a linear phase front.
The Ruze lens, as described by John Ruze in "Wide-Angle Metal-Plate Optics", Proceedings of the I.R.E., Vol. 38, No. 1, Jan. 1950, pages 53-59, is an example of a lens beam forming network. The Ruze lens is a line source antenna that can provide multiple, independently steerable, simultaneous beams. Like other microwave lenses, it has a focal arc with each position along that arc corresponding to a different beam direction. Pointing the beam in a particular direction is accomplished by merely placing a beam launching device at the corresponding location on the focal arc of the lens and scanning of the beam is accomplished by moving the beam launcher along the focal arc. Using multiple beam launchers produces multiple simultaneous beams each of which may be steered independently of the other beams. In addition, the aperture of the lens can be large enough to produce the desired far field beamwidth independent of the number of resolvable beam directions that are used.
One type of beam launcher is a waveguide. Each independent beam requires its own length of waveguide. Changing the direction of any of the multiple simultaneous beams produced by the waveguide beam launchers requires the mechanical relocation of the waveguide.
An alternative to the waveguide beam launcher is an array of monopole elements, hereinafter referred to as probes, or radiating elements mounted along the focal arc. Each probe location corresponds to a specific beam direction. When driven by an electromagnetic energy source, a probe will radiate energy in a well defined and predetermined direction. And, since the lens is a reciprocal device, energy received from that direction will come to a focus at that probe.
Beam pointing angles corresponding to locations between two adjacent probes can be achieved by splitting the power from the electromagnetic source between the two adjacent probes and by amplitude and/or phase weighting of the distributed power.
Typically, a complex network of switches directing signals to the probes on the focal arc is used to achieve rapid and random steering of beams. The switch network is nominally the same kind of switch network that would be required to switch between input ports of a Butler matrix or any other matrix beam forming network. As with the matrix beam forming networks, in many applications, the switching network must be capable of handling high power levels.
The Ruze lens is only one of many lens antennas wherein the beam direction corresponds to a location on the focal arc. Other examples of lenses include, but are not limited to, the Rotman lens as described by W. Rotman and R. F. Turner in "Wide-Angle Microwave Lens for Line Source applications", IEEE Transactions on Antennas and Propagation, Vol. AP-16, No. 6, Nov. 1963, pages 623-632; the Luneburg lens as described on pages 189 through 213 in "Mathematical Theory of Optics," published by Brown University in 1944; and other lenses such as the R-2R and R-KR lenses described by D. H. Archer in "Lens-Fed Multiple Beam Arrays", Microwave Journal, Sep. 1984, pages 171-195.